Present day telephone cords are made with individually insulated conductors disposed in a planar array and enclosed in a common jacket which comprises a plasticized polyvinyl chloride (PVC) composition. Each conductor is generally made with a center filamentary core having a plurality of tinsel ribbons wrapped spirally thereabout and is highly flexible relative to a solid or stranded wire conductor. Modular plugs such as those shown in U.S. Pat. Nos. 3,699,498 and 3,860,316, for example, are used to terminate the cords so that they can be easily connected to modular jacks within telephone housings or wall terminals.
A cord which is used to connect a base or housing and a handset portion of a telephone is referred to as a retractile or spring cord and is made from a length of cordage which is helically wound in a plurality of convolutions while that which is used to connect the housing to a wall terminal is called a line cord and comprises a length of unwound cordage. In recent years, a substantial number of telephone cords, both retractile and line, have been made with a clear, as opposed to a colored, opaque jacket.
In order to improve the resistance of telephone cords, especially the clear ones, to contaminants, to wear, and to unsightly staining during use, the PVC is coated with a plastic barrier which comprises a polyester composition that is applied to the top or external surface of the cord jacket. A polyester plastic which results in a superior external coating is disclosed and claimed in application Ser. No. 864,159 filed Dec. 27, 1977 in the names of W. I. Congdon, J. J. Mottine and W. C. Vesperman, and incorporated by reference hereinto.
Several problems have surfaced during the manufacture and use of jacket-coated cords and, in particular, of jacket-coated, retractile cords. While the top coating stiffens and prevents contamination of the cord jacket, the repeated extension and retraction of the cord causes the coating to be lifted away from the jacket and to crinkle. The crinkling of the coating, which occurs because the stiffness modulus of the coating material is substantially greater than that of the jacket, results in an unsightly rippled jacket surface and neutralizes the aesthetic value provided by the clear jacket.
Because of the extension and retraction of the convolutions of a retractile cord during customer use, the cord cross-section tends to assume an oval configuration rather than one with the conductors in a planar array. It is important that the conductors be disposed and maintained in a planar array in a predetermined orientation; otherwise, end sections of the cordage may not mate with receiving openings in modular terminating plugs such as, for example, those described in aforementioned U.S. Pat. Nos. 3,699,498 and 3,860,316 which issued on Oct. 17, 1972 and Jan. 14, 1975, respectively. Also, when used cords are refurbished, any reorientation of the conductors would cause discontinuities upon retermination of the cordage with plugs.
Another problem relates to the symmetry of the cord cross section with respect to x-y coordinate axes and, in particular, to the jacket wall thickness adjacent the outermost conductors in the array. A lack of control of wall thickness as the jacket is extruded over the array results in cordage which, if used to make retractile cords, is susceptible to splitting. Further, insufficient or unsymmetrical jacket wall thickness, especially with respect to the outermost conductors, results in a defect called herniation in which a protrusion develops in the outer surface of the jacket.
Unsymmetrical jacket wall thickness is caused by variation in the outside diameter of one or more of the insulated conductors. Plastic for conductor insulation, such as that shown, for example in U.S. Pat. No. 4,090,763, which issued May 23, 1978 in the names of W. I. Congdon, J. J. Mottine and W. C. Vesperman, is extremely sensitive to temperature change and will vary in size in the absence of suitable temperature control. Also, imperfections in the tinsel ribbon conductors make it more difficult to control the outside diameter of the insulation that that of a solid wire.
The non-uniformity of the thickness of the jacket on the side of each of the outermost conductors in the planar array is also caused by the shifting of the relatively flexible, tinsel ribbon conductors in a cavity of the extruder between the core tube and die because of flow imbalance conditions in the plastic material about the array. These conditions which are characterized, for example, by differences in pressure are discussed in U.S. Pat. Nos. 2,766,480, 2,778,059 and 3,860,686 with respect to a single conductor. The problem of flow imbalance is magnified when an array of relatively flexible conductors is to be jacketed.
The lateral shifting of the conductors can be controlled somewhat by the use of an individual core tube passageway for each conductor. However, in the event of a conductor break prior to entry into the core tube, the unbroken conductors as well as the leading portion of the broken conductor would continue to be advanced, thereby permitting plastic material to enter the passageway associated with the broken conductor. Restringing of the broken conductor would require that the manufacturing line be shut down so that the associated core tube can be cleaned. On the other hand, if the conductors are advanced through a single passageway core tube wherein they are arrayed contiguously and a conductor break occurs, the broken conductor would be moved along through the remainder of the core tube by frictional engagement with the unbroken ones. This permits a restringing operation to be carried out during a brief period of reduced line speed instead of requiring a complete shut down of the line. Although the use of a single passageway core overcomes one problem relating to line breaks, the conductors 21--21 arrayed contiguously are susceptible to lateral shifting which results in cordage having an unsymmetrical jacket.
The known prior art includes U.S. Pat. No. 919,384 which shows a plurality of cables in a planar array with a separator between at least two adjacent cables and stitched to the jacket. In U.S. Pat. No. 2,659,932, a ground strand, which is interposed between two parallel, insulated conductors, is covered, after which the conductors and covered ground strand are enclosed with an insulating material. An apparatus shown in U.S. Pat. No. 4,050,867, is used during extrusion to maintain a constant distance between wires or optical fibers of a telephone cable by causing the wires or fibers to be moved through an extrusion head along precisely defined paths. In these last two mentioned patents, a plastic material is formed between the conductors prior to their enclosure in a common jacket.
The prior art also includes a product known as drop wire which connects outside telephone plant to a customer's premises. In the manufacture of drop wire, two relatively stiff, copper-clad, steel wires are advanced through separate passageways in a core tube of an extruder with the spacing between the passageways being substantially the same as the spacing between the wires after they have passed through the extruder die.
The prior art does not seemingly offer a solution to the problem of how to produce a telephone cord which includes a plurality of relatively flexible conductors that are arrayed in a single layer during manufacture, maintained in such a configuration during use, and enclosed with a substantially symmetrical, coated jacket.